The Global Positioning System (GPS) comprises 24 satellites in low earth orbit that continually broadcast their position and local time. Through satellite range measurements, a terrestrial (or airborne) receiver can determine its absolute position as long as four satellites are within view.
Galileo is Europe's initiative for a state-of-the-art Global Navigation Satellite System (GNSS), providing a highly accurate, guaranteed global positioning service. According to the European white paper “European transport policy for 2010: time to decide”, GNSS is identified as a critical technology. Galileo shall be designed and developed using time, geodesy and signal structure standards interoperable and compatible with civil GPS and its augmentations. In the near to medium-term future the market for satellite navigation technology is expected to experience major growth.
Portable, consumer GNSS receivers require solutions that are compact, cheap and low-power e.g. have long battery life. To enable widespread proliferation of GNSS capabilities into consumer products, an integrated receiver should minimize the number of off-chip components. Integration of the entire receiver will minimize the part-to-part variation of discrete receivers. An integrated receiver will be easier to reproduce from product to product, since the precise layout of hundreds of components is not required. Furthermore, it is easier to manufacture and attain desired yield because the function of the entire receiver has been verified at the chip level. These considerations have lead to research into and deployment of new receiver architectures utilising low-IF or zero-IF (direct-conversion) approaches (IF: Intermediate Frequency). However, although providing high-level of integration and elimination of off-chip components these architectures suffer from IQ-phase and gain impairments, resulting in limited image-rejection that can be achieved. This is hindering their wide-spread economical use in commercial products.
The same problems are set to continue except more challenging with the significantly different modulation format Galileo has when compared to all previous GNSS systems. This is defined in the document “Status of Galileo Frequency and Signal Design (25.09.2002)”, Guenter W. Hein, et al, http://europa.eu.int/comm/dgs/energy_transport/galileo/documents/technical_en .htm; as well as in working paper “GALILEO Signals: RF Characteristics (ICAO NSP/WGW: WP/36)—http://www.galileoju.com. Galileo includes three signal bands namely: E5, E6 and L1 with respective centre frequencies of 1191.795 MHz, 1278.750 MHz, and 1575.420 MHz. The E5 band contains two signals E5a and E5b. Galileo satellites will transmit signals in the E5a band (1176.450 MHz) and E5b band (1207.14 MHz) as a composite signal with a centre frequency of 1191.795 MHz. Modulation of the E5 will be Alternate Binary Offset Carrier (AltBOC). The generation of this signal is described in above two references. Referring to Appendix A of “Status of Galileo Frequency and Signal Design”, a standard Binary Offset Carrier (BOC) modulation uses a rectangular subcarrier which can be a sine or a cosine of frequency fs, e.g. sign(sin(2πfst)), to modulate a time domain signal s(t). This shifts the frequency of the signal to both upper sideband and corresponding lower sideband. BOC type signals are usually expressed in the form BOC(fs,fchip) where fs is the rectangular sub-carrier frequency and fchip is the spreading code chipping rate. Frequencies are indicated as multiples of 1.023 MHz. For example a BOC(10,5) signal has actually a sub-carrier frequency of 10×1.033 MHz=10.330 MHz and a spreading code chipping rate of 5×1.023 MHz=5.115 MHz. AltBOC on the other hand uses complex rectangular sub-carrier which is complex exponential given as sign(ej(2πfst)). Using the Euler formula this can be written as sign[cos(2πfst)+j sin(2πfst)]. In this way the signal spectrum is not split up, but only shifted to higher frequencies. Shifting to lower frequencies is also possible. The goal of the AltBOC modulation is to generate in a coherent manner the E5a and E5b bands which are respectively modulated by complex exponentials or sub-carriers, such that signals can be received as a wideband BOC-like signal. Constellation diagram for AltBOC modulated signals are given in FIG. 1.
The L1 signal consists of the multiplexing of three components that are L1P channel, L1F data channel and L1F pilot channel whereas the E6 signal consists of the multiplexing of E6p and E6c. These signals on the E6 and L1 bands use Coherent Adaptive Sub-carrier Modulation (CASM) which is also referred to as Interplex or Modified Tricode Hexaphase to generate the composite signals. This is defined in “Tricode Hexaphase Modulation for GPS”, Proceedings of Institute of Navigation (ION)-GPS Annual Meeting, pages: 385-397, 1997 and in “L1 band part of Galileo Signal in Space ICD (SIS ICD)/also referred as: Galileo standardisation document for 3GPP”—http://www.galileoju.com. With this modulation format a QPSK signal resulting from the combination of two channels is phase modulated with the third channel. The modulation index m=0.6155 is used to set the relative power between the three channels. Constellation diagram for CASM/modified Hexaphase modulated signals are given in FIG. 2.
E5 is one of the most advanced and promising signals the Galileo satellites will transmit. Galileo receivers capable of tracking this signal will benefit from unequalled performance in terms of measurement accuracy, indoor performance and multipath suppression. However, the signal processing techniques required to process the AltBOC modulation are much more challenging than those for the traditional BPSK or even for the conventional BOC modulation. This stems from the extremely large bandwidth and from the complex interaction of the components in the spreading code.
As indicated above, Galileo receivers will suffer from IQ phase and gain impairments. Quadrature modulation and demodulation systems modulate data onto in-phase (I) and quadrature (Q) components of a baseband signal and then mix those baseband signals with I and Q components of a Radio Frequency (RF) carrier to broadcast the modulated data. The Q signal is ninety degrees out of phase with the I signal. In the receiver the reverse process is carried out, first receiving the broadcast signal, then downconverting to recover the I and Q components of the modulated baseband signal, and then recovering the data from those I and Q components.
Receiver architectures that utilize IQ-signal processing are vulnerable to mismatches between the I and Q channels. This can happen at several stages in the receiver; the RF splitter used to divide the incoming RF signal equally between the I and Q paths may introduce phase and gain differences. The differences in the length of the two RF paths can result in phase imbalance. The quadrature 90° phase-splitter used to generate the I and Q Local-Oscillator (LO) signals that drive the I and Q channel mixers may not be exactly 90°. Furthermore, there might be differences in conversion losses between the output ports of the I and Q channel mixers. In addition to these, filters and Analog-to-Digital-Converters (ADCs) in the I and Q paths are not perfectly matched. The effects of these impairments on the receiver's performance can be detrimental. The IQ-imbalances can be characterized by two parameters: the amplitude mismatch, αε and the phase orthogonality mismatch, φε between the I and Q branches. The amplitude-imbalance, β in decibels is obtained from the amplitude mismatch αε as:β=20 log10[1+0.5αε/1−0.5αε]  (1)
The Quadrature Demodulator receiver model of FIG. 3 incorporates IQ-imbalances as impaired LO signals. An input signal s(t) is mixed with a local oscillator signal fLO in quadrature channels. The mixed signal is subject in each channel to gain and Low Pass Filtering (LPF).
FIG. 4 demonstrates the effects of varying the IQ phase and gain mismatches on the raw Bit-Error-Rate (BER) performances of the systems using (a) 32-PSK and (b) 256-QAM modulation formats. As can be observed the IQ-imbalances degrade the system's BER performance greatly. This degradation in performance is not desirable and must be compensated. In order to ensure correct symbol detection RF impairments must be compensated for before the symbol decision takes place.
In the papers “Adaptive Compensation of Analog Front-End I/Q Mismatches in Digital Receivers”, Cetin E., Kale I., Morling R. C. S., IEEE International Symposium on Circuits and Systems, (ISCAS 2001), vol. 4, pp. 370-373, May 2001. , “Adaptive Self-Calibrating Image Rejection Receiver”, Cetin E., Kale I., Morling R. C. S., IEEE International Conference on Communications (ICC 2004), vol. 5, pp. 2731-2735, June 2004. , “On the structure, convergence and performance of an adaptive I/Q mismatch corrector” by: Cetin, E.; Kale, I.; Morling, R. C. S., IEEE Vehicular Technology Conference (VTC 2002 Fall), vol. 4, pp. 2288-2292, September 2002, there is discussed single ended zero-IF and Low-IF I/Q channel wireless systems. The papers propose a blind (unsupervised) technique that does not require pilot tones, but instead employs a blind adaptive algorithm. It is recognized that mismatch errors create cross-correlation between the I and Q channels in the case of zero-IF approach or the desired and the adjacent/interfering channel in the case of low-IF approach. In order to remove the cross-correlation, adaptive filters are cross-coupled between the I and Q channels. The coefficients of the filters are updated by the adaptive algorithm chosen. The signals that were subject of these papers were relatively simple to process as compared with the highly complex wideband schemes of Galileo, as explained above.